MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors), in particular power MOSFETs, are widely used as electronic switches for switching electrical loads or as electronic switches in all types of switching converters. A power MOSFET includes a drain region, a drift region adjoining the drain region, and a source region, each having a first conductivity type, and a body region arranged between the drift region and source region of a second conductivity type. A gate electrode serves to control a conducting channel in the body region between the source region and the drift region. The source region is electrically connected to a source electrode which is also connected to the body region, and the drain region is electrically connected to a drain electrode.
A MOSFET can be operated in forward biased state and in a reverse-biased state. In the forward biased state, a voltage between the drain and the source electrode is such that a P-N junction between the body region and the drift region is reverse biased. In the forward biased state the MOSFET can be switched on and off by applying a suitable electrical potential at the gate electrode. In the reverse biased state of the MOSFET the p-n junction between the body region and the source region is forward biased, so that the MOSFET in the reverse biased state acts like a diode, commonly known as a body diode.
In many applications, such as applications in which a MOSFET is used as a switch that cyclically switches an inductive load, there are time periods in which the MOSFET is reverse biased so that the body diode conducts a current. The losses that occur when a current flows through the MOSFET in its reverse direction are dependent on the current and the forward voltage of the body diode. The forward voltage of the body diode is the voltage that is required to bias the body diode such that it conducts a current. In a silicon MOSFET the forward voltage is about 0.7V.
There is a need to provide a MOSFET that has reduced losses when it is operated to conduct a current in its reverse direction.